The multi-strand superconducting conductors used in windings or in alternators of the cryoalternator type are often made in the form of a flat tapes over whose surfaces the superconductor strands are twisted for transposition purposes, so that the strands constantly form two layers one on each side of the tape, each strand passing alternately from one layer to the other round the edges of the tape. When high voltages are induced by variable magnetic fields, the short circuits which are then the most likely to occur are those between the layers at the points where the strands pass from one to the other.
The first necessity which then appears is to dispose a separator in the form of a tape between the two layers, with at least the surface of the tape acting as an insulator. Indeed, it is known that such a separator alone is sufficient to provide practical protection against short-circuits between the layers.
The second necessity which appears in numerous cases is to detect rapidly transistions in which a strand changes locally from the superconducting state to the normal state, so as to then switch to prevent such transition from extending along the conductor and leading finally to the destruction thereof by overheating. Such transitions are detected by the voltage of resistive origin which they set up across terminals of the conductor. However, this resistive voltage can be masked by much higher voltages which are induced along the length of the conductor by the variable magnetic fields. Therefore, to detect the resistive voltage, it is necessary to remove the effect of the induced voltage. In the case of a winding, it is known that this can be done by providing thereon a centre tap and putting the total voltages which appear in the two halves of the winding in opposition so that the induced voltages compensate each other while the resistive voltage which results from an accidental local transition normally appears only across the terminals of one of the halves of the winding only and can therefore be detected. This solution is not applicable to the windings of an alternator.
Another known solution consists in adding a "detector" superconductor line along the entire length of the "main" superconductor line. The detector line is in contact with the main line via an insulating layer. The same voltages are induced in the detector (which is left open circuit) as in the main line. Thus to detect the resistive voltage it is then only necessary to compare the voltages in the two conductors. This solution has the drawback of complicating the construction of the conductor.
Preferred embodiments of the present invention provide a flat multi-strand superconductor having a separator, its construction is simple and it facilitates detection of any transitions, even when used in the windings of an alternator.